1. Field of the Invention
The invention relates to an apparatus for the corrective positioning of a travelling sheet or web at right angles to the direction of travel. The term "travelling sheet or web" is to be understood generally as any kind of product that is processed as a semi-finished product or, in an intermediate manufacturing stage, as sheet material, such as plastic sheet, plastic bags, newspapers, imitation leather, paper webs in the case of corrugated board production etc. On the other hand, this term is also meant to comprise continuously circulating belts utilized for instance as so-called "wire cloth" or "felt belts" in paper machines or as conveyor belts in materials-handling technology.
2. Background Art
The sheets as well as belts of the generic type tend to escape laterally during machine operation. For this to be prevented, so-called "sheet guiders" are employed, which ensure a clearly defined position of the running sheets. In this connection, three substantial sources of errors in the running of a sheet are to be cited:
static errors conditioned by badly adjusted deflection rolls, laterally misaligned sheet supply rolls etc. in a sheet-processing machine PA1 dynamic errors conditioned by wavy edges, wobbling rolls, humidity, temperature fluctuations etc. PA1 tension errors provoked by irregular distribution of tension over the cross-section of the sheet.
Various embodiments of so-called sheet guiders are known from the prior art. For instance, so-called rotating frames are used, in which two guide rolls that are parallel to each other at a distance in the direction of sheet travel are disposed in a frame which is rotatable about a vertical axis by a corresponding servo component for instance in the form of a pneumatic cylinder. In this way, the two guide rolls can be inclined slightly relative to the direction of travel on a horizontal plane so that a corresponding transverse displacement is conferred to the sheet running over the guide rolls. The transverse position of the sheet can be corrected permanently by adjustment of the rotating frame.
According to further known prior art, a sheet guiding system is used for the guide control of continuous belts such as conveyor belts and wire cloth belts of varying materials, for instance plastic material, rubber, metal fabric, textile fabric, etc., making use of a sheet-guiding roller having an actuator, a thrust bearing, and a mechanical tracing system in the form of a roller lever. On one side the sheet guiding roller lodges pivotably in the thrust bearing by way of its bearing pins, while rotarily sliding in a combination bearing on the other side. The combination bearing is again mounted on an actuator which is disposed laterally of the sheet to be guided and by means of which the sheet-guiding roller, on the other side, is pivotal about the thrust bearing that forms its pivot. The pivoting motion is controlled by the edge of the sheet being mechanically traced.
Both prior art sheet guiders have the drawback of fundamentally needing active control by an edge tracer and a servo unit.
This also applies to the system disclosed by DE 195 17 960 A1 for the travel control of a belt in a corrugated-board machine for so-called "single-face corrugated board". In this case, a continuous belt runs over a plurality of rolls, one of which is rotatably run on a support shaft. One end of this support shaft is pivotably housed in an articulation point, while its opposite end is housed in a ball-and-socket joint which is displaceable by means of a spindle drive at right angles to the direction of rotation of the roll. The belt travel is again traced by a detecting device and the axis of rotation of the roll is adjusted by operation of the actuator in such a way that warping of the belt is precluded.
The professional paper "Wochenblatt fur Papierfabrikation", February 1965, page 94, discloses an automatic guide roll--called "servo roll"--for wire cloth and felt belts in paper machines, which does not need a position sensor for the belt to be guided nor any actively operating servo control component. Rather, the guide roll compensates any escape motion of the belt to be guided by tilting motion, returning same into its desired position.
To this end, the guide roll is equipped with a special tilting bearing system. The latter comprises a stationary central pipe, which centrally exhibits an approximately perpendicular bolt. This bolt constitutes a tilting axis about which a shorter piece of pipe may reciprocate in the horizontal. This piece of pipe has a ball bearing at each of its ends, about which can rotate an exterior sleeve pipe of glass fiber reinforced polyester. This sleeve pipe is the actual guide roll.
By it being specifically profiled to have a slightly conical taper at the end and owing to the explained tilting bearing arrangement, the guide roll will incline and will slightly right simultaneously when the belt to be guided escapes to one side. As a result, the belt is guided back to the middle and the guide roll takes a position precisely perpendicular to the direction of travel.
A drawback of the prior art guide roll resides in the fact that the tilting bearing system is comparatively hidden inside the sleeve pipe, therefore being difficult to mount. Moreover, the force resulting from the described bearing behavior concentrates on the center of the central pipe to be borne so that for increased working widths, the roll must have very great diameters for reasons of stability. Furthermore, this guide roll does not offer any possibility of additionally providing an active positioning system, if the reaction velocity of the automatic positioning system does not comply with the respective application.